CIECULATION 



267 



the pressure falls. This fall in pressure is indicated by the 

 dicrotic notch. But the elasticity of the seinilunar valves at 

 ouce makes them again spring up, thus increasing the pres- 

 sure in the aorta and causing the second crest, the dicrotic 

 wave. After this the pressure in the arteries steadily 

 diminishes till the mean is reached, to be again increased 

 by the next ventricular systole. 



The form of the pulse wave varies according to the relation- 

 ship between the arterial pressure and the activity of the 

 heart. 



If the heart is active and strong in relation to the arterial 



FIG. 124. Diagram to show the relationship of the pulse wave to the cardiac 

 cycle and the effect of altering the relationship between the activity of the 



heart and the arterial blood pressure. b is the curve of iiitra- 



ventricular pressure, and b l is a pulse curve with an active heart 



and a relatively low arterial pressure. a and a 1 are the same with a 

 sluggish heart and a relatively high arterial pressure. 



pressure, the main mass of the blood is expelled in the first 

 sudden outflow, and the residual flow is absent or slight 

 (fig. 124, dotted line). In this case there is a sudden and 

 marked rise of the arterial pressure, followed by a steady fall 

 till the moment of ventricular diastole. The rebound of the 

 semilunar valves is marked in this case and causes a very 

 prominent dicrotic wave, while the predicrotic wave is absent 

 (fig. 123, i). Such a condition is well seen after violent 

 muscular exertion, and in certain fevers. In these conditions 

 the dicrotic wave is so well marked that it can be readily felt 

 with the finger. It is to this form of pulse that the term 

 dicrotic is applied in medicine. 



